The present invention relates to a method of manufacturing semiconductor devices. More particularly, the present invention relates to a method of manufacturing flash memory devices.
Non-volatile memory devices are configured to store information even when they are not applied with power unlike volatile memory devices. Accordingly, the non-volatile memory devices such as flash memory devices have been widely used in a file system, a memory card, a portable device, and so on.
The non-volatile memory device has a stack gate structure in which the tunnel oxide film, the floating gate, the dielectric film and the control gate electrode are sequentially stacked on the channel region of the semiconductor substrate.
The non-volatile memory cell with the stack gate structure can be programmed by implanting hot electrons. In other words, a high voltage is applied to the control gate and a potential difference is created between the source and drain regions. As a result, hot electrons are generated in the channel region near the drain. These hot electrons acquire sufficient energy to overcome the energy barrier of the tunnel oxide film and are injected into the floating gate. If the electrons are injected into the floating gate, the threshold voltage of the transistor (or memory cell) is increased. If a voltage applied to the control gate is less than the increased threshold voltage, the transistor remained turned-off and current does not flow through the cell. The above process is used to store and read information from the non-volatile device.
Information stored in the non-volatile memory cell having the stack gate structure can be erased by discharging the electrons that have been injected into the floating gate by F-N tunneling phenomenon. In other words, the source region is applied with a high voltage, the control gate electrode and the substrate are applied with 0V, and the drain is floated. Consequently, a strong electric field is generated between the source region and the floating gate, thereby causing the F-N tunneling to occur.
Since the floating gate is formed using a conductive film, almost all of the electrons injected into the floating gate can be removed near the source region by the F-N tunneling during the erase operation.
However, the non-volatile memory cell having the stack gate electrode structure may have a problem related to electron maintenance. In other words, in order for the non-volatile memory cell to maintain the programmed information, the non-volatile memory cell must maintain electrons injected into the floating gate electrode. If defects such as pin holes exist in the tunnel oxide film, however, the electrons injected into the floating gate electrode may exit through the defects. Since the floating gate is formed of the conductive film, a significant leakage current is generated by the defects in the tunnel dielectric film.
In addition, too many electrons may be discharged from the floating gate during the erase operation since the floating gate is formed of the conductive film and the electrons move freely therein.